Welding processes using lasers, such as high power continuous wave (CW) CO.sub.2 lasers, for example, require nearly constant energy deposition at the weld to produce acceptable welds. With robotic or automated laser welding systems which are required to function over a wide travel range such as an extended length weld on a large work piece, divergence of the laser beam as it travels from the laser generator in one location to the workpiece at a relatively remote location, will result in changes in the focal spot diameter at the weld. This effect results from the larger, diverged beam diameter at the focusing optics which have a fixed focal length. The f number, which is defined as the focal length of the focusing optics divided by the beam diameter, decreases as the beam diameter increases, thereby changing the focusing of the beam. Since the power density is a function of the square of the focal spot diameter, the welding parameters are very sensitive to the beam diameter.
Laser beams produced by commerically available, high power (12 to 25 kW) continuous wave CO.sub.2 lasers, for example, are about 2 to 3 inches (0.051 to 0.076 m) in nominal diameter. These size beams are subject to enlarging significantly in diameter in accordance with their transport over large distances from the laser generator to the welding station. We recognize, however, that larger beams, say about 5 to 7 inches (0.13 to 0.18 m) in diameter, will diverge proportionately less over the same distances.
It is the aim of this invention to utilize these phenomena to control the ultimate size of the focal spot at the welding location irrespective of significantly different distances of travel of the laser beam.